A) Field of the Invention
The present invention relates to a semiconductor optical amplifier and a light amplification method, and more particularly to a semiconductor optical amplifier which induces optical amplification upon flowing current and introducing light into an active layer containing quantum dots, quantum wires or quantum dashes and to a light amplification method.
B) Description of the Related Art
For the cost reduction of metro optical networks and access optical networks, it is expected that coarse Wavelength Division Multiplexing (WDM) having a wide inter-channel wavelength (about 20 nm) will be widely used. Coarse WDM does not require precise control over the wavelengths of light sources and filters and permits a wavelength fluctuation to some degree. The cost reduction is therefore possible because it is not necessary to use a cooler for controlling the temperature of a semiconductor laser used as a light source, or from other reasons.
The wavelength range actually used is from 1.3 μm to 1.6 μm. A single conventional optical amplifier having a bandwidth of about 0.05 μm cannot cover the whole wavelength range actually used. If an optical amplifier is to be prepared for each usable wavelength band, this requirement contradicts the cost reduction.
Japanese Patent Laid-open Publication No. 2002-344090 discloses a semiconductor optical amplifier having quantum dots. Quantum dots are used in an active layer of the semiconductor optical amplifier so that the volume of the active layer can be reduced and a carrier density in the active layer at a limited current density can be increased. Since the carrier density can be increased, a gain spectrum can be broadened.
Another related art is disclosed in Japanese Patent Laid-open Publication No. HEI-5-267795.
Although the gain spectrum can be broadened by using quantum dots in the active layer of a semiconductor optical amplifier, the gain is largely dependent upon the wavelength. This semiconductor optical amplifier is difficult to be used as a broadband optical amplifier used in coarse WDM.